An age old problem in the metal casting industry is the presence in the casting of stresses which distort the shape of the casting and can cause brittle fracture or stress corrosion in the part. Up until the last two decades, the only known effective way to stress relieve the part was by thermal stress relief. This entailed loading the part or parts into an oven or onto a conveyor traveling through an oven where the part or parts were heated to elevated temperatures for an extended period of time and cooled also over an extended period of time. The extent of the heating and cooling and the length of time involved is dependent upon the type of material, size of the part, intricacy of the part and the like.
The effect of thermal or heat treatment is dimensional stability and stress reduction but it can significantly alter the metallurgy of the part and can cause scaling and discoloration. The altered metallurgy will generally avoid brittle fracture, stress corrosion and creep.
Thermal treatment of material is expensive, requiring large consumption of fuel, substantial outlay for equipment and floor space and a long time for effecting the process. The expenditure is particularly wasteful where the only stress needed to be treated is in the field of shape stabilization.
A system of vibratory stress relief was devised within the last two decades where castings can be shape stabilized by inducing one or more vibrating states using a high force exciter. The high force exciter causes the structure to undergo overall elastic distortion such as might be obtained by loading it mechanically, except that the loading modes induced by vibration are generally more complex and variable than could be achieved by externally applied static forces. The manually applied strains cause regions of internal stress to give rise to local plasticity where the sum of the internal and vibratory induced strains exceed the yield point. The plasticity causes a redistribution and reduction in internal stress. The existent vibratory stress relief equipment use AC or DC vibrators which operate from 0 to 200 Hz and apply a full frequency range to the part, which part is isolated from the ground on resilient pads. The equipment has been successful in reducing costs, eliminating scaling and discoloration and resulted in the parts being stress relieved within the field of shape stabilization. However, the equipment does not clean the casting during the application of the stress relieving vibratory force.